Photonic sintering of polymer thick film conductor compositions

ABSTRACT

This invention provides a method for using a polymer thick film conductor composition to form an electrical conductor in an electrical circuit, the method subjecting the deposited thick film conductor composition to photonic sintering. The invention also provides a method for reducing the resistance of an electrical conductor formed from a polymer thick film conductor composition, the method comprising the step of subjecting the electrical conductor to photonic sintering. The invention further provides devices containing electrical conductors made by these methods.

FIELD OF THE INVENTION

The invention is directed to the photonic curing of polymer thick film(PTF) conductor compositions for use in many different applications. Inone embodiment, the PTF conductor composition is used as ascreen-printed conductor on a thin film substrate such as ITO-sputteredglass. The PTF conductor functions as a grid electrode. This compositionmay further be used for any other application where conductivity (lowresistivity) is required.

BACKGROUND OF THE INVENTION

This invention is directed to a polymer thick film conductor compositionfor use in electronic devices. PTF silver conductors are quite prevalentin electronic circuitry as the conductor of choice due to the lowresistivity (<50 milliohms/sq) and the reliability of silver. However,in recent years, the price of silver has tripled to the order of morethan $30/troy oz and it is therefore becoming expensive to use incircuitry. Alternatives to silver are being sought with littlecompromise in electrical properties but at reduced cost. It is thepurpose of this invention to provide such an alternative.

SUMMARY OF THE INVENTION

This invention provides a method for forming an electrical conductor inan electrical circuit, comprising:

-   -   a) providing a substrate;    -   b) providing a polymer thick film conductor composition selected        from the group consisting of a polymer thick film solder alloy        conductor composition, a polymer thick film solder alloy/metal        conductor composition, and mixtures thereof;    -   c) applying the thick film conductor composition onto the        substrate; and    -   d) subjecting the thick film conductor composition to photonic        sintering to form the electrical conductor.

In an embodiment, the method further comprises a step of drying thethick film conductor composition, wherein said step is carried outfollowing step (c) but before step (d). The composition may be processedat a time and temperature necessary to remove all solvent. Further,photonic sintering after drying reduces the resistivity by 70-80%.

In one embodiment, the polymer thick film conductor composition is apolymer thick film solder alloy conductor composition comprising:

-   -   (a) 65 to 95 wt % solder alloy powder consisting of tin; silver,        and copper and possessing an average particle size of 2 to 18 μm        and a surface area/mass ratio in the range of 0.2 to 1.3 m²/g;        dispersed in    -   (b) 5 to 35 wt % organic medium comprising        -   (i) a vinyl co-polymer resin of vinylidene chloride and            acrylonitrile, dissolved in            -   (2) organic solvent comprising a dibasic ester; wherein                the wt % are based on the total weight of the polymer                thick film solder alloy conductor composition.

In another embodiment, the polymer thick film conductor composition is apolymer thick film solder alloy/metal conductor composition comprising:

-   -   (a) 35 to 94 wt % solder alloy powder selected from the group        consisting of (i) a tin, silver, and copper alloy powder, (ii) a        tin and bismuth alloy powder, and (iii) mixtures thereof, said        alloy powder consisting of particles possessing an average        particle size of 2 to 18 and a surface area/mass ratio in the        range of 0.2 to 1.3 m²/g;    -   (b) 1 to 30 wt % metal selected from the group consisting of        silver, copper, gold, aluminum, and mixtures thereof, said metal        consisting of particles possessing an average particle size of 2        to 18 μm and a surface area/mass ratio in the range of 0.1 to        2.3 m² g; and    -   (c) 5 to 35 wt % organic medium comprising:        -   (1) a resin that is a vinyl co-polymer resin of vinylidene            chloride and acrylonitrile or a phenoxy resin; dissolved in        -   (2) organic solvent comprising a dibasic ester or glycol            ether;            with the proviso that if said resin is a phenoxy resin said            metal is silver; wherein said solder alloy powder and said            metal are dispersed in said organic medium and wherein the            wt % are based on the total weight of the polymer thick film            conductor composition.

The invention also provides an electrical device comprising anelectrical conductor formed by any of the embodiments of the abovedescribed method.

The invention further provides a method for reducing the resistance ofan electrical conductor formed from a polymer thick film conductorcomposition, the method comprising the step of subjecting the electricalconductor to photonic sintering. It also provides an electrical devicecomprising such an electrical conductor.

DETAILED DESCRIPTION OF INVENTION

Generally, a thick film composition comprises a functional phase thatimparts appropriate electrically functional properties to thecomposition. The functional phase comprises electrically functionalpowders dispersed in an organic medium that acts as a carrier for thefunctional phase. Generally, in thick film technology, the compositionis fired to burn out the organics and to impart the electricallyfunctional properties. However, in the case of polymer thick films, theorganics remain as an integral part of the composition after drying.“Organics” as used herein comprise polymer, resin or binder componentsof a thick film composition. These terms may be used interchangeably andthey all mean the same thing.

In one embodiment, the polymer thick film conductor composition is apolymer thick film solder ahoy conductor composition comprising a SAC(tin, silver, copper) alloy powder dispersed in an organic mediumcomprising a polymer resin and a solvent.

In another embodiment, the polymer thick film conductor composition is apolymer thick film solder alloy/metal conductor composition comprising aSAC alloy powder, a Sn/Bi alloy powder or a mixture of both and a metalselected from the group consisting of silver, copper, gold, aluminum,and mixtures thereof, wherein the solder alloy powder and the metal aredispersed in an organic medium comprising a polymer resin and a solvent.

To summarize, the main components of the polymer thick film conductorcompositions are conductor powders dispersed in an organic medium, whichis comprised of polymer resin and solvent. The polymer thick filmconductor compositions used in two embodiments are discussed below.

Thick Film Solder Alloy Conductor Composition Embodiment

A. Conductor Powder

The electrically functional powder in the thick film solder alloyconductor composition is a solder alloy conductor powder that containstin, silver, and copper, known as SAC alloy, with tin the largestcomponent, i.e. greater than 90% by weight.

The particle diameter and shape used in the solder alloy powder areparticularly important and have to be appropriate to the applicationmethod. In one embodiment the particles are spherical. In anotherembodiment the particles are in flake form. The particle sizedistribution of the solder alloy particles is also critical with respectto the effectiveness of the invention. As a practical matter, theparticle size is in the range of 1 to 100 μm. In an embodiment, theaverage particle size is 2 to 18 μm. In addition, the surfacearea/weight ratio of the solder alloy particles is in the range of0.2-1.3 m²/g.

Furthermore, it is known that small amounts of one or more other metalsmay be added to the solder alloy conductor compositions to improve theproperties of the conductor. Some examples of such metals include: gold,silver, copper, nickel, aluminum, platinum, palladium, molybdenum,tungsten, tantalum, tin, indium, lanthanum, gadolinium, boron,ruthenium, cobalt, titanium, yttrium, europium, gallium, sulfur, zinc,silicon, magnesium, barium, cerium, strontium, lead, antimony,conductive carbon, and combinations thereof and others common in the artof thick film compositions. The additional metal(s) may comprise up toabout 1.0 percent by weight of the total composition.

An organic acid may be used as a reductant for the solder alloy toreduce any oxidation of the solder alloy surface.

B. Organic Medium,

The solder alloy powder is typically mixed with an organic medium(vehicle) by mechanical mixing to form a paste-like composition, called“paste”, having suitable consistency and rheology for printing. Theorganic medium must be one in which the solids are dispersible with anadequate degree of stability. The rheological properties of the organicmedium must be such that they lend good application properties to thecomposition. Such properties include: dispersion of solids with anadequate degree of stability, good application of composition,appropriate viscosity, thixotropy, appropriate wettability of thesubstrate and the solids, a good drying rate, and dried film strengthsufficient to withstand rough handling.

The organic medium comprises a solution of polymer in organicsolvent(s). The organic medium is not conventional in the art and lendsunique properties to the composition.

The resin used in the present embodiment is a vinyl co-polymer ofvinylidene chloride and acrylonitrile which allows high weight loadingof solder alloy powder and thus helps achieve both good adhesion tosubstrates and low resistivity (high conductivity), two criticalproperties for conductors in electronic circuitry. Additionally, thispolymer seems to act as a self-fluxing component in the pastes so noexternal reductant is needed.

A wide variety of inert liquids can be used as solvents in the organicmedium. The most widely used solvents found in thick film compositionsare ethyl acetate and terpenes such as alpha- or beta-terpineol ormixtures thereof with other solvents such as kerosene, dibutylphthalate,butyl carbitol, butyl carbitol acetate, hexylene glycol and high boilingalcohols and alcohol esters. In addition, volatile liquids for promotingrapid hardening after application on the substrate can be included inthe vehicle. In many embodiments of the present invention, solvents suchas glycol ethers, ketones, esters and other solvents of like boilingpoints (in the range of 180° C. to 250° C.), and mixtures thereof may beused. In one embodiment the organic medium is based on dibasic estersand C-11 ketone solvent. Various combinations of these and othersolvents are formulated to obtain the viscosity and volatilityrequirements desired.

Thick Film Solder Alloy/Metal Conductor Composition Embodiment

A. Conductor Powders

The electrically functional powder in the thick film solder alloy/metalconductor compositions is (1) solder alloy conductor powder selectedfrom the group consisting of tin, silver, and copper alloy powders,known as SAC alloys, with tin the largest percentage, i.e. greater than90% by weight, Sn/Si alloy powders with at least 40 wt % tin andmixtures thereof and (2) metal powders/flakes selected from the groupconsisting of silver, copper, gold, aluminum or mixtures thereof.

The particle diameter and shape used on both the solder alloy powdersand pure metal are particularly important and have to be appropriate tothe application method. The particle size distribution of both thesolder alloy particles and pure metal is also critical with respect tothe effectiveness of the invention. As a practical matter, the particlesize is in the range of 1 to 100 μm. In an embodiment, the averageparticle size of both the solder alloy and the metal is 2 to 18 μm. Inaddition, the surface area/weight ratio of the solder alloy particles isin the range of 0.2-1.3 m²/g, while that of the metal is 0.1-2.3 m2/g.Although silver and copper are the two preferred metal particles, thisembodiment is not limited to those two metals and others such as goldand aluminum could be used.

An organic acid may be used as a reductant for the solder alloy toreduce any oxidation of the solder alloy surface, although it is notrequired here.

B. Organic Medium

The powders are typically mixed with an organic medium (vehicle) bymechanical mixing to form a paste-like composition, called “paste”,having suitable consistency and rheology for printing. The organicmedium must be one in which the solids are dispersible with an adequatedegree of stability. The rheological properties of the organic mediummust be such that they lend good application properties to thecomposition. Such properties include: dispersion of solids with anadequate degree of stability, good application of composition,appropriate viscosity, thixotropy, appropriate wettability of thesubstrate and the solids, a good drying rate, and dried film strengthsufficient to withstand rough handling.

The organic medium comprises a solution of polymer in organicsolvent(s). The organic medium is not conventional in the art and lendsunique properties to the composition.

The polymer resin of the present embodiment is particularly important.The resin of choice used in the present invention is a vinyl co-polymerof vinylidene chloride and acrylonitrile which allows high weightloading of solder alloy powder and thus helps achieve both good adhesionto substrates and low resistivity (high conductivity), two criticalproperties for conductors in electronic circuitry. Additionally, thispolymer seems to act as a self-fluxing component in the pastes so noexternal reductant is needed. Alternatively, a phenoxy resin may be usedin some formulations as well, but only when silver is the added metal.

A wide variety of inert liquids can be used as solvents in the organicmedium. The most widely used solvents found in thick film compositionsare ethyl acetate and terpenes such as alpha- or beta-terpineol ormixtures thereof with other solvents such as kerosene, dibutylphthalate,butyl carbitol, butyl carbitol acetate, hexylene glycol and high boilingalcohols and alcohol esters. In addition, volatile liquids for promotingrapid hardening after application on the substrate can be included inthe vehicle. In many embodiments of the present invention, solvents suchas glycol ethers, ketones, esters and other solvents of like boilingpoints (in the range of 180° C. to 250° C.), and mixtures thereof may beused. In one embodiment the organic medium is based on dibasic estersand C-11 ketone. Various combinations of these and other solvents areformulated to obtain the viscosity and volatility requirements desired.

Application of Thick Films

The polymer thick film conductor compositions also known as “pastes” aretypically deposited on a substrate, e.g., ITO-sputtered glass, that isessentially impermeable to gases and moisture. The substrate can also bea sheet of flexible material. An example is an impermeable plastic suchas a composite material made up of a combination of plastic sheet withoptional metallic or dielectric layers deposited thereupon. Thesubstrate must be such as to withstand the processing temperature of140° C. In one embodiment, the substrate can be a build-up of layerswith metalized solder alloy paste.

The deposition of the polymer thick film solder alloy/metal compositionsis performed preferably by screen printing, although other depositiontechniques such as stencil printing, syringe dispensing or coatingtechniques can be utilized. In the case of screen-printing, the screenmesh size controls the thickness of deposited thick film.

Photonic Sintering

Photonic sintering uses light to provide high-temperature sintering.Typically, a flash lamp is used to provide the source of light and isoperated with a short on time of high power and a duty cycle rangingfrom a few hertz to tens of hertz. The photonic sintering step is brief,typically less than 1 minute. Photonic sintering may also be referred toas photonic curing.

The photonic sintering of the deposited thick film conductor compositionprovides conductors with low resistivity, i.e., as low as less than 10milliohm/sq.

In one embodiment, prior to the photonic sintering, the deposited thickfilm conductor composition is dried by exposure to heat at lowtemperatures, typically for 10-15 min at 140° C. This is achieved withan initial drying temperature approximately 80° C. below the liquidustemperature 217° C. of one of the solder alloys (SAC305).

The present invention will be discussed in further detail by givingpractical examples. The scope of the present invention, however, is notlimited in any way by these practical examples.

EXAMPLE AND COMPARATIVE EXPERIMENT Comparative Experiment A

A PTF solder alloy metal conductor composition was prepared by mixingsolder alloy powder SAC305 (AMTECH, SMT International LLC, Deep River,Conn.) with an average spherical particle size of 10 μm (range was 5-15μm) and silver flake with an average particle size of 5 microns with anorganic medium composed of a co-polymer of vinylidene chloride andacrylonitrile resin (Saran™ F-310 resin, Dow Chemical Co., Midland,Mich.). The molecular weight of the resin was approximately 25,000. Asolvent was used to dissolve the resin completely prior to adding thesolder alloy powder. That solvent was a 50/50 blend of dibasic esters(DuPont, Wilmington, Del.) and Eastman™ C-11 Ketone solvent (EastmanChemical, Kingsport, Tenn.).

The composition of the PTF solder alloy/metal conductor composition isgiven below:

58.25% SAC305 solder alloy powder - (96.5% Sn, 3% Ag, 0.5% Cu) 9.71Organic medium (19.5% resin/80.5% solvent) 29.12 Silver Flake (averagesize 5 microns) 0.92 Carbitol Acetate 2.00 DiBasic Esters

This composition was mixed for 10 minutes in a Thinky-type mixer. Thecomposition was used to screen print a pattern on glass. Using a 280mesh stainless steel screen, a series of lines were printed, and thesolder alloy paste was dried at 140° C. for 10 min. in a forced air boxoven. The resistivity was then measured as 35 milliohm/sq at a thicknessof 30 microns.

Example 1

The same PTF solder alloy/metal composition was essentially prepared andprinted as described in Comparative Experiment A, except that thecomposition was subjected to photonic sintering after drying. All otherproperties of the formulation, solder alloy powder distribution, and thesubsequent processing were the same as Comparative Experiment A.

The photonic sintering was performed on a Hoist Flash Sintering Unit,(Hoist Centre, Eindhoven, NL) in three stages: stage 1: 16.66 Hz, 60%power. stage 2: 7.14 Hz, 50% power. stage 3: 2.5 Hz, 100% power. Thetotal time was 30 sec. The resistivity for this composition wasapproximately 9 milliohm/sq, which is substantially reduced from the 35milliohm/sq obtained with oven drying only in Comparative Experiment A.It is apparent that the photonic sintering has a beneficial effect uponthe resistivity of the polymer thick film conductor composition.

What is claimed is:
 1. A method for forming an electrical conductor inan electrical circuit, comprising: A) providing a substrate; B)providing a polymer thick film conductor composition, wherein saidpolymer thick film conductor composition is a polymer thick film solderalloy conductor composition comprising: (a) 65 to 95 wt % SAC solderalloy powder consisting of tin, silver, and copper, wherein said tin isgreater than 90 wt % of said SAC solder alloy powder, said SAC solderalloy powder consisting of particles possessing an average particle sizeof 2 to 18 μm and a surface area/mass ratio in the range of 0.2 to 1.3m²/g; dispersed in (b) 5 to 35 wt % organic medium comprising (1) avinyl co-polymer resin of vinylidene chloride and acrylonitrile,dissolved in (2) organic solvent comprising a dibasic ester; wherein thewt % of said SAC solder alloy powder and said organic medium are basedon the total weight of the polymer thick film solder alloy conductorcomposition; C) applying said polymer thick film conductor compositiononto said substrate; and D) subjecting said polymer thick film conductorcomposition to photonic sintering to form said electrical conductor. 2.The method of claim 1, said method further comprising a step of dryingsaid thick film conductor composition, wherein said step of drying iscarried out following step (c) but before step (d).
 3. A method forforming an electrical conductor in an electrical circuit, comprising: A)providing a substrate; B) providing a polymer thick film conductorcomposition, wherein said polymer thick film conductor composition is apolymer thick film solder alloy/metal conductor composition comprising:(a) 35 to 94 wt % solder alloy powder selected from the group consistingof (i) SAC solder alloy powder consisting of tin, silver, and copperalloy powder, wherein said tin is greater than 90 wt % of said SACsolder alloy powder, (ii) solder alloy powder consisting of tin andbismuth alloy powder, wherein said tin is at least 40 wt % of said tinand bismuth alloy powder, and (iii) mixtures thereof, said ahoy powderconsisting of particles possessing an average particle size of 2 to 18μm and a surface area/mass ratio in the range, of 0.2 to 1.3 m²/g; (b) 1to 30 wt % metal selected from the group consisting of silver, copper,gold, aluminum and mixtures thereof, said metal consisting of particlespossessing an average particle size of 2 to 18 μm and a surfacearea/mass ratio in the range of 0.1 to 2.3 m²/g; and (c) 5 to 35 wt %organic medium comprising: (1) a resin that is a vinyl co-polymer resinof vinylidene chloride and acrylonitrile or a phenoxy resin; dissolvedin (2) organic solvent comprising a dibasic ester or glycol ether;wherein if said resin is a phenoxy resin said metal is silver: whereinsaid solder alloy powder and said metal are dispersed in said organicmedium and wherein the wt % of said solder alloy powder, said metal andsaid organic medium are based on the total weight of the polymer thickfilm conductor composition; C) applying said polymer thick filmconductor composition onto said substrate; and D) subjecting saidpolymer thick film conductor composition to photonic sintering to formsaid electrical conductor.
 4. The method of claim 3, said method furthercomprising a step of drying said thick film conductor composition,wherein said step of drying is carried out following step (c) but beforestep (d).
 5. A method-for reducing the resistance of an electricalconductor formed from a polymer thick film conductor composition, saidmethod comprising the step of subjecting the electrical conductor tophotonic sintering, wherein said polymer thick film conductorcomposition is a polymer thick film solder alloy conductor compositioncomprising: (a) 65 to 95 wt % SAC solder alloy powder consisting of tin,silver, and copper, wherein said tin is greater than 90 wt % of said SACsolder alloy powder, said SAC solder alloy powder consisting ofparticles possessing an average particle size of 2 to 18 μm and asurface area/mass ratio in the range of 0.2 to 1.3 m²/g; dispersed in(b) 5 to 35 wt % organic medium comprising (1) a vinyl co-polymer resinof vinylidene chloride and acrylonitrile, dissolved in (2) organicsolvent comprising a dibasic ester; wherein the wt % of said SAC solderalloy powder and said organic medium are based on the total weight ofthe polymer thick film solder alloy conductor composition.
 6. A methodfor reducing the resistance of an electrical conductor formed from apolymer thick film conductor composition, said method comprising thestep of subjecting the electrical conductor to photonic sintering,wherein said polymer thick film conductor composition is a polymer thickfilm solder alloy/metal conductor composition comprising: (a) 35 to 94wt % solder alloy powder selected from the group consisting of (i) SACsolder alloy powder consisting of tin, silver, and copper alloy powder,wherein said tin is greater than 90 wt % of said SAC solder alloypowder, (ii) solder alloy powder consisting of tin and bismuth alloypowder, wherein said tin is at least 40 wt % of said tin and bismuthalloy powder, and (iii) mixtures thereof, said ahoy powder consisting ofparticles possessing an average particle size of 2 to 18 μm and asurface area/mass ratio in the range, of 0.2 to 1.3 m²/g; (b) 1 to 30 wt% metal selected from the group consisting of silver, copper, gold,aluminum and mixtures thereof, said metal consisting of particlespossessing an average particle size of 2 to 18 μm and a surfacearea/mass ratio in the range of 0.1 to 2.3 m²/g; and (c) 5 to 35 wt %organic medium comprising: (1) a resin that is a vinyl co-polymer resinof vinylidene chloride and acrylonitrile or a phenoxy resin; dissolvedin (2) organic solvent comprising a dibasic ester or glycol ether;wherein if said resin is a phenoxy resin said metal is silver: whereinsaid solder alloy powder and said metal are dispersed in said organicmedium and wherein the wt % of said solder alloy powder, said metal andsaid organic medium are based on the total weight of the polymer thickfilm conductor composition.